The present invention relates to a zeolite adsorbent exchanged with barium and calcium cations, to a gas purification process using such an adsorbent, in particular an air pre-treatment prior to its separation by cryogenic distillation, and to its manufacturing process.
It is known that atmospheric air contains compounds that have to be removed before the air is introduced into heat exchangers of the cold box of an air separation unit, for example carbon dioxide (CO2) and/or water vapor (H2O).
This is because, in the absence of such a pre-treatment of the air to remove the CO2 and water vapor impurities therefrom, these impurities can condense and solidify as ice during cooling of the air to cryogenic temperature, hence resulting in problems of equipment blockage, especially in heat exchangers, distillation columns, etc.
Furthermore, it is also desirable to remove the hydrocarbon impurities likely to be present in the air so as to avoid any risk of damaging the equipment.
It is preferable also to remove the nitrogen oxides likely to be found in the air, such as N2O, so as to prevent them from being concentrated and deposited in the reboilers of the cryogenic distillation plants, with the risk of blocking them.
Currently, this air pre-treatment is carried out by adsorption using, depending on the case, a TSA (Temperature Swing Adsorption) process or a PSA (Pressure Swing Adsorption) process.
TSA air purification processes have been described for example in documents U.S. Pat. No. 3,738,084 and FR-A-7 725 845.
In general, the CO2 and water vapor (H2O) impurities are removed over one or more beds of adsorbents, preferably several beds of adsorbents, namely in general a first adsorbent intended to preferentially stop water, for example a bed of activated alumina, of silica gel or zeolites, and a second bed of adsorbent for preferentially stopping CO2, for example a zeolite. This is because effective removal of CO2 and water vapor contained in the air over one and the same bed of adsorbent is not easily accomplished, as water has a markedly greater affinity for the adsorbents than CO2 has, and it is therefore standard practice to use at least two beds or layers of adsorbents of different type.
In this regard, mention may, for example, be made of the documents U.S. Pat. Nos. 5,531,808, 5,587,003 and 4,233,038.
In the document “zeolite molecular sieves”, Krieger Publishing Company, 1984, page 612, D. W. Breck recommends the use of an unexchanged 13X-type zeolite (sodium form) to remove small amounts of CO2 and possibly of water as it has a strong affinity and selectivity for these polar molecules.
However, the 13X zeolite does not make it possible to stop, in a manner equal to or better than CO2, all the harmful molecules likely to be present in a gas stream, in particular hydrocarbons and nitrogen oxides, as recalled by the following: E. Alpay, “Adsorption parameters for strongly adsorbed hydrocarbon vapours on some commercial adsorbents”, Gas Sep. & Purif., Vol. 10, No. 1, pp. 25 (1996); G. Calleja, “Multicomponent adsorption equilibrium of ethylene, propane, propylene and CO2 on 13X zeolite”, Gas Sep. & Purif., Vol. 8, No. 4, p. 247 (1994); V. R. Choudhary, “Sorption isotherms of methane, ethane, ethene and carbon dioxide on NaX, NaY and Na-mordenite Zeolites”, J. Chem. Soc. Faraday Trans., 91 (17), p. 2935 (1995); and A. Cointot, P. Cartaud and C. Clavaud, “Etude de l'adsorption du protoxyde d'azote par différents tamis moléculaires”, [Study of nitrous oxide adsorption by various molecular sieves]”, Journal de Chimie Physique, Vol. 71, No. 5, p. 765–770 (1974).
It therefore follows that an up-stream industrial air purification unit strictly designed for stopping carbon dioxide using a standard zeolite, typically a 13X or 5A zeolite, stops only partly, or even not at all, ethylene, propane, other hydrocarbons and nitrous oxide, as recalled in Dr J. Reyhing's document “Removing hydrocarbons from the process air of air-separation plants using molecular-sieve adsorbers”, Linde Reports on Science and Technology, 36/1983.
As regards stopping nitrous oxide, the ineffectiveness of the 5A zeolite for stopping N2O compared with CO2 was demonstrated by U. Wenning in “Nitrous oxide in air separation plants”, MUST'96, Munich Meeting on Air Separation Technology, 10–11 Oct. 1996.
One solution was proposed in document EP-A-1 064 978, which discloses an adsorbent consisting of an X or LSX (low-silica X) zeolite exchanged to at least 30%, preferably at least 75%, with barium cations, which zeolite can be used to remove certain impurities from the air, in particular nitrous oxide, propane and ethylene, the residual cations being sodium and/or potassium cations.
The zeolites disclosed therein are obtained by an ion exchange process which is quite complex as soon as the degree of exchange with barium has to exceed 50%.
This is because a zeolite usually consists of a negatively charged aluminosilicate framework in which compensating cations occupy positions defined by the charge of the cation, by its size and polarizability, and by the charge of the zeolitic framework and its crystalline structure.
According to that document, the exchanged zeolite is obtained by ion exchange starting with an X or LSX zeolite initially containing sodium (Na+), to end up with a zeolite containing at least 30% barium.
However, Ba2+ cations are voluminous and cannot reach certain crystallographic sites occupied by the Na+ cations, this having the effect of limiting the degree of exchange to about 75% at most.
To reach higher values (>75%), it is necessary to perform additional operations intended to force the cations to migrate towards the barely accessible sites. The applicable procedure consists in carrying out a first exchange with barium and then in drying the zeolite and heating it to at least 200° C. The Ba2+ cations are then stripped of their train of solvation water molecules and, moreover, they are subjected to greater thermal agitation. Migration towards the inaccessible sites can then take place.
It should be noted that these sites are thermodynamically favored and that only steric hindrance prevents the cations from occupying them.
Moreover, it seems that it is the accessible sites, i.e. the II and II′ sites, which give the barium cations their remarkable properties.
The adsorbent disclosed in EP-A-1 064 978 cannot therefore be regarded as completely satisfactory from the technical standpoint—the many hydrothermal treatments to which it has to be subjected damage its structure—and it is also very expensive compared with the adsorbents currently used because of the high amount of barium that has to be used to perform the ion exchange.
From there, the problem that then arises is to be able to have a zeolite-type adsorbent which is approximately as effective for removing hydrocarbons and, if possible, more effective for removing nitrogen oxides in a gas stream to be purified, in particular air, but which is easier to manufacture and therefore of lower cost than that known from EP-A-1 064 978.
The object of the present invention is therefore to try to solve this problem by providing an improved zeolite adsorbent that can be used to purify gases, such as air, and its manufacturing process.